1. Field of the Invention
This invention relates to a method and apparatus for cleaning a substrate with an aqueous solution of ammonium fluoride as a cleaning solution, and more specifically, to a novel substrate cleaning method and apparatus developed with objectives of providing a uniform and stable cleaning process with less amount of cleaning solution consumed.
2. Description of the Related Arts
Aqueous solutions of ammonium fluoride hydrofluoric acid mixture and ammonium fluoride or hydrofluoric acid are used as a cleaning solution for cleaning a semiconductor substrate, a glass substrate in the process of manufacturing a semiconductor and a LCD (Liquid Crystal Display). The semiconductor substrate is cleaned mainly for the purpose of removing an oxide layer (a thermal oxide layer or a native oxide layer) from the substrate surface by etching, while the glass substrate is cleaned for the purpose of removing a surface layer from the glass substrate similarly by etching. Incidentally, the above aqueous solution of ammonium fluoride is a desired hydrofluoric acid ammonium mixture solution, which is dissolved in the form of NH4F in water and sometimes contains a surfactant.
In the technical field of manufacture of the semiconductor and the LCD, the art capable of finer processing is required for higher integration to provide more lightweight, smaller-sized, less power consumption products. In view of the above circumstances, there has been a need for a more accurate cleaning process in the substrate cleaning art using the above cleaning solution as well.
Incidentally, wet-cleaning of the semiconductor substrate in the related art is generally by the process of storing a cleaning solution in a cleaning bath provided in a cleaning draft, loading a number of semiconductor substrates in a container called a wafer cassette, and then immersing the substrates inclusively of the cassette in the cleaning solution for cleaning, while allowing clean air to flow through the cleaning draft without the need for special control.
In this related art, the HF (hydrofluoric acid) concentration in the cleaning solution increases with the lapse of cleaning time as shown in FIG. 1. Incidentally, FIG. 1 is a graphical representation of the relation between the elapsed time (cleaning time) since it started to use the cleaning solution for the cleaning process and the HF concentration of the cleaning solution. In consequence, the cleaning process in the above related art is of no effect on uniform and stable cleaning by reason of the considerably non-uniform change in concentration rise as well as the increase in etching rate of the silicon oxide or the glass substrate surface layer with the lapse of time as shown in FIG. 2, resulting in problems inclusive of difficulty in providing higher yield of the semiconductor substrate and the liquid crystal display substrate.
The rise of HF concentration of the cleaning solution with the lapse of cleaning time may occur due to the following causes. That is, one of the causes is that a water content exhaled from the cleaning solution is discharged to the outside of the cleaning draft together with clean air. Another cause is that ammonium hydroxide and hydrofluoric acid are dissociating from ammonium fluoride (NH4F) in the aqueous solution of ammonium fluoride so that ammonium hydroxide is discharged in the form of ammonium gas to the outside of the cleaning draft similarly together with the clean air.
For that reason, the rise of active component concentration of the cleaning solution (inclusively of non-uniform concentration) has been controlled by means of frequent replacement of the cleaning solution in the related art. However, a considerably large amount of cleaning solution is required for performing the above measure. In particular, since the ammonium fluoride cleaning solution is used at the concentration as high as about 40 wt. %, there is a need for a larger amount of chemicals consumed per cycle of solution replacement, as compared with other cleaning solutions normally used at the concentration of about several wt. %.
A waste fluid treatment (a waste water treatment) according to a process as shown in FIG. 3 is also required for the used hydrofluoric acid cleaning solution (a hydrofluoric acid cleaning solution 5). However, the waste fluid treatment in this case brings about not only the need for a large amount of resources consumed, that is, waste water treatment agents but also the increase in released waste (waste water and sludge) amount with the increasing amount of resources consumed, as is apparent from a material balance view of FIG. 4. Incidentally, FIG. 3 is a flow sheet (a flow of the treatment for an ammonium fluoride waste fluid released from the semiconductor manufacture process) showing the process of the waste fluid treatment for the used cleaning solution. FIG. 4 is a view illustrating a material balance relating to the process of the waste fluid treatment in FIG. 3.
Incidentally, the global environmental issues have been recently made as a matter of worldwide concerns, and influences of the semiconductor or LCD substrate manufacture process on the environment have been at issue. In these days, there has been socially a long-felt need for measures not only to decrease the cleaning cost in the substrate manufacture process but also to meet the preservation of the environment inclusive of the resource saving, the decrease in released waste amount and the environmental purification.
For that reason, an apparatus having a cleaning bath installed under the atmosphere subjected to temperature, humidity and clean air flow or like control has been proposed as a cleaning apparatus structured to permit the chemical composition of a cleaning solution in the cleaning bath to be maintained at a value within a predetermined range without the need for frequent replacement of the cleaning solution (See Japanese Patent Laid-open No. 9-22891). However, this cleaning apparatus, although uses a humidified air curtain, has difficulty in maintaining the cleanliness of substrates to be cleaned by reason of the horizontal flow of clean air, and besides, needs to discontinue the flow of air every operation of taking in and out the substrates to be cleaned, resulting in problems inclusive of the need for troublesome operations.
As described above, the substrate wet-cleaning process in the related art is also of no effect on uniform and stable cleaning by reason of the considerably non-uniform change in concentration rise as well as the increase in etching rate of the silicon oxide layer (or the glass substrate surface) with the lapse of cleaning time (that is, with the elapsed time since it started to use the cleaning solution for the cleaning process), resulting in problems inclusive of difficulty in providing higher yield of the semiconductor substrate and the liquid crystal display substrate.
As the result of measurement conducted by the present inventor as to the HF (hydrofluoric acid) concentration of the above cleaning solution, it was ascertained that the HF concentration of the cleaning solution increases with the lapse of cleaning time as shown in FIG. 6, and the etching rate of the silicon oxide increases with the increasing HF concentration of the cleaning solution on the basis of the results shown in FIGS. 5 and 6, as shown in FIG. 7. It was also ascertained that the cleaning apparatus having the cleaning bath installed in the cleaning draft allowing the clean air to flow therethrough for exhaust of air shows a particularly marked tendency toward the increase in etching rate.
The rise of HF concentration of the cleaning solution with the lapse of cleaning time occurs from the following causes. That is, one of the causes is that a water content is exhaled from the cleaning solution. Another cause is that ammonium hydroxide and hydrofluoric acid are dissociating from ammonium fluoride (NH4F) in the aqueous solution of ammonium fluoride so that the ammonium hydroxide is exhaled in the form of ammonium gas.
For that reason, the rise of active component concentration of the cleaning solution (inclusively of the non-uniform concentration) has been controlled by means of frequent replacement of the cleaning solution. However, a considerably large amount of cleaning solution is required for the above means. In particular, since the ammonium fluoride cleaning solution is available at the concentration as high as about 40 wt. %, there is a need for a larger amount of ammonium fluoride and hydrofluoric acid consumed per cycle of solution replacement, as compared with other cleaning solutions normally available at the concentration of about several wt. %.
A waste fluid treatment (a waste water treatment) according to the process as shown in FIG. 3 is also required for the spent hydrofluoric acid cleaning solution (the hydrofluoric acid cleaning solution). However, the waste fluid treatment in this case brings about not only the need for a large amount of resources consumed, that is, waste water treatment agents but also the increase in released waste (waste water 35, 45 and sludge 36, 46) amount with the increasing amount of resources consumed, as is apparent from a material balance view of FIG. 8. Incidentally, FIG. 8 is a view illustrating a material balance relating to the process of the waste fluid treatment in FIG. 3.
In Japanese Patent laid-open JP No. 9-22891 (the invention titled “Apparatus and method for wet process”), there is disclosed a wet process apparatus having an effect on a uniform etching/cleaning process from the viewpoint of the lapse of time. This wet process apparatus has two tanks, that is, a chemical composition tank and a composition control tank, whereby composition control chemicals are supplied from the composition control tank to a chemical bath (an etching bath and a cleaning bath) arranged at a predetermined location to maintain the composition-changed chemicals in the chemical bath at the predetermined composition.
However, the composition control tank is merely suitable for storage of the composition control chemicals supplied to the chemical bath, and the composition control chemicals are merely available for composition control. The above disclosed apparatus is fundamentally different in these points from the substrate wet-cleaning method and apparatus according to the present invention as will be described later, and shows the operations and effects also quite different therefrom.
The process of cleaning or etching the substrate with the aqueous solution of ammonium fluoride or ammonium fluoride hydrofluoric acid mixture presents problems inclusive of the change (increase) in etching rate for the silicon oxide or the glass substrate by reason of the change (evaporation) in chemical components (NH4F and HF) or water content in the cleaning solution with the lapse of time.
The above problem is caused by the gradual increase in concentration of hydrofluoric acid component in the cleaning solution with the lapse of time in the process of being used. On the other hand, in the existing circumstances, frequent replacement of the cleaning solution has been applied as a measure to cope with the concentration change in various components in the cleaning solution.
However, a considerably large amount of cleaning solution is required for the above measure (In particular, since the ammonium fluoride cleaning solution is used at the concentration as high as several ten wt. % (about 40 wt. %, for instance), there is a need for a larger amount of chemicals consumed per cycle of solution replacement, as compared with other cleaning solutions normally available at the concentration of about several wt. %.), resulting in the increase in ammonium fluoride or hydrofluoric acid consumption (chemical cost) with the increasing amount of cleaning solution used.
The waste water treatment as shown in FIG. 3 is also required for the used cleaning solution. However, the waste water treatment in this case brings about not only the need for a large amount of resources (waste water treatment agents) consumed but also the increase in released waste (waste water and sludge) amount with the increasing amount of resources consumed (See FIG. 4).
FIG. 3 is a view illustrating the process of the waste fluid treatment for the used cleaning solution. As shown in FIG. 3, the waste fluid discharged from the cleaning bath 1 is carried to a pH control tank 2 for neutralization with a 20% calcium hydroxide solution. Subsequently, the neutralized waste fluid is carried to a coagulo-sedimentation tank 3 for coagulo-sedimentation with chemicals such as aluminum sulfate, and is further carried to a coagulation tank 4 for coagulation and precipitation with a polymeric coagulant such as a polyacrylic amide coagulant, and finally, the resultant is released as sewage and sludge.
In the above process of the waste fluid treatment as shown in FIG. 4, 2.0 Kg of 20% calcium hydroxide, 0.3 Kg of 8% aluminum sulfate and 1.6 Kg of polymeric coagulant are used for 1.0 Kg of an aqueous solution of 40% ammonium fluoride, and 2.6 Kg of 70% solid sludge and 2.3 Kg of waste water are released as wastes.
Incidentally, the global environmental issues have been recently made as a matter of worldwide concerns, and the environmental burden on the semiconductor or LCD substrate manufacture process has been at issue. In these days, there has been socially a long-felt need also on the cleaning solution for measures not only to decrease the cost but also to meet the preservation of the environment inclusive of the resource saving, the decrease in released waste amount and the environmental purification.